Ureas As Herbicides

[unigirl]

I having problems establishing the toxicity pathways of Ureas in perennial weeds and pre and post emergent broadleaved weeds and grasses. Is anyone aware of how they function?

Thank you

[Calibax]

Inhibition of acetolactate synthase (ALS)
According to this: http://en.wikipedia.org/wiki/Acetolactate_synthase
Inhibitors of ALS like sulfonylureas are used as herbicides that slowly starve affected plants of common amino acids, which eventually leads to inhibition of DNA synthesis.

[theprof]

Hmm, a challenging question at last. OK, first of all, the ureas are inhibitors of the Hill reaction inhibiting photosynthesis by binding to the QB binding niche on the D1 protein of the photosystem 11 complex in chloroplast thylakoid membranes. Electron transport from QA to QB is prevented and fixation of CO and production of ATP and NADPH is halted.

Phytotoxicity also results from the formation of triplet state chlorophyll and its interaction with ground-state oxygen to form singlet oxygen resulting from the plant's inability to reoxidise QA. Triplet state chlorophyll and singlet oxygen can abstract hydrogen from unsaturated lipids and initiate lipid peroxidation in a chain reaction. Chlorophyll and carotenoids are lost through leaking membranes because lipids and proteins are oxidised.

A general rule is that herbicidal selectivity is reduced by substitution in the phenyl ring with halogens especially in the 4 and 2 positions as in diuron. Selectivity is increased by alkyl substitution in the phenyl ring such as in chlorotoluron and isoproturon because metabolic hydroxylation can occur on those substituents.

Essentially, crop selectivity is correlated with the metabolic balance between N- emethylation/demethoxylation and hydroxylation with subsequent glucosylation of the aromatic ring or its side chain.

[theprof]

That's an interesting link Calibax. I'm afraid it has very little to do with ALS and certainly nothing to do with DNA synthesis. The writer of that Wikipedia entry was way off the mark and it's very misleading. Thank you all the same.

[theprof]

Ha! I've just discovered that Wikipedia has no direct article on the Hill Reaction. I always thought Wikipedia knew everything!! Time to get typing people.

[unigirl]

That's great. Thanks again prof. I think I'm on the right track at last!

[theprof]

Hmm, I would have thought that as Calibax has provided a Wikipedia link in the post methyl, then it should have been obvious that the text contained in the post matched that in the Wikipedia entry, even if it was from elsewhere on Wikipedia. I fail to see the point here. It's not unusual for AB users to cut and paste text with the best interests.

Turning to post 2 (ie my post), the nearest phrase I can see to anything in my post on the Herbicide page on Wikipedia that you mention is "They bind to the Qb site on the D1 protein, and prevent quinone from binding to this site". Regretfully methyl, the quoted phrase cannot be regarded as precisely the same as anything I've posted. Examine my post closely: do you appreciate the inclusion of the word "niche"? I also seem to have omitted quinone in my post which seems to eliminate plagiarism on my part. What are you getting at?

Incidentally, just like the quote provided by Calibax, the entries under the sub-headings "ALS inhibitors" and "Photosystem 11 Inhibitors" in the link you provide methyl are fundamentally misleading and are very far from the truth. It resembles something written by a first year biochemistry undergraduate with insufficient knowledge of plant biochemistry. The attributed toxicity mechanisms are wrong.